Default assignment of scheduled transmissions

ABSTRACT

Methods and apparatuses are presented for conducting communications over a shared communication medium, involving (a) sending a request from a first node, the shared communication medium organized to include (i) a request signal space including request segments each having a different location within the request signal space and (ii) a scheduled transmission signal space including scheduled transmission segments each having a different location within the scheduled transmission signal space, the request sent in a request segment, (b) obtaining an assignment, in a first tier of assignments, associating the request with a scheduled transmission segment, (c) from the first node, sending a data transmission in the scheduled transmission segment associated with the request, and (d) the plurality of scheduled transmission segments including at least one default use segment available for use by a default entity if the at least one default use segment is not assigned in the first tier of assignments.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application claims benefit of priority under 35 U.S.C.119(e) of U.S. provisional Application No. 60/888,451, filed on Feb. 6,2007, entitled “Symbol Oriented Reservation Request System ForControlling Access To A Shared Communications Link,”the content of whichis incorporated herein by reference in its entirety.

The present application also claims benefit of priority under 35 U.S.C.119(e) of U.S. provisional Application No. 60/895,143, filed on Mar. 16,2007, entitled “Blip and Burst Multiple Access,”the content of which isincorporated herein by reference in its entirety.

The following U.S. nonprovisional patent applications, including thepresent application, are being filed concurrently and the disclosure ofevery other application is incorporated by reference in the presentapplication in its entirety for all purposes:

-   -   U.S. Nonprovisional patent application Ser. No. 11/771,762,        filed Jun. 29, 2007, entitled “Reservation Request System for        Controlling Access to a Shared Communication Medium.”    -   U.S. Nonprovisional patent application Ser. No. 11/771,798,        filed Jun. 29, 2007, entitled “Time Multiplexed Requests for        Controlling Access to a Shared Communication Medium.”    -   U.S. Nonprovisional patent application Ser. No. 11/771,810,        filed Jun. 29, 2007, entitled “Code Multiplexed Requests for        Controlling Access to a Shared Communication Medium.”    -   U.S. Nonprovisional patent application Ser. No. 11/771,828,        filed Jun. 29, 2007, entitled “Contention and Polled Requests        for Scheduling Transmissions.”    -   U.S. Nonprovisional patent application Ser. No. 11/771,840,        filed Jun. 29, 2007, entitled “Request Signal Designs for        Multiple Service Types.”    -   U.S. Nonprovisional patent application Ser. No. 11/771,856,        filed Jun. 29, 2007, entitled “Default Assignment of Scheduled        Transmissions.”    -   U.S. Nonprovisional patent application Ser. No. 11/771,870,        filed Jun. 29, 2007, entitled “Successive Scheduled Requests for        Transmission.”    -   U.S. Nonprovisional patent application Ser. No. 11/771,882,        filed Jun. 29, 2007, entitled “Piggyback Requests in Scheduled        Transmissions.”    -   U.S. Nonprovisional Patent Application No. 11/771,894, filed        Jun. 29, 2007, entitled “Robust and Efficient Assignment of        Scheduled Transmissions.”    -   U.S. Nonprovisional Patent Application No. 11/771,903, filed        Jun. 29, 2007, entitled “Request Detection Error Processing.”    -   U.S. Nonprovisional patent application Ser. No. 11/771,910,        filed Jun. 29, 2007, entitled “Assignment Modes for a Shared        Communication Medium.”    -   U.S. Nonprovisional patent application Ser. No. 11/771,926,        filed Jun. 29, 2007, entitled “Reservation Request        Improvements.”

BACKGROUND OF THE INVENTION

In many applications, a communication medium is shared among a number ofnodes. The nodes compete with one another for access to the sharedcommunication medium. At any given moment, there may be more than one ofthe nodes that wish to transmit data over the shared communicationmedium. A system is typically put in place to facilitate access to theshared communication medium by the various nodes. Various categories ofsuch multiple access systems have been developed.

One category of multiple access systems utilizes contention protocols.Examples of these contention protocols include the ALOHA protocol andthe slotted ALOHA protocol, which are known in the art. Here, each nodeis allowed to freely transmit its data over the shared communicationmedium at any time or any slotted time. In a system employing a hub,each node sends its transmission to the hub, which then broadcasts thetransmission to all nodes. In a system without a hub, each node directlybroadcasts its transmission to all nodes. In either case, every nodelistens to the channel for its own transmission and attempts to receiveit. If a node is unsuccessful in receiving its own transmission, thenode can assume that its transmission was involved in a collision withanother transmission, and the node simply re-transmits its data afterwaiting a random amount of time. In this manner, collisions are allowedto occur but are resolved by the nodes.

Another category of multiple access systems utilizes carrier senseprotocols. Examples include persistent carrier sense multiple access(persistent CSMA) and non-persistent carrier sense multiple access(non-persistent CSMA) protocols, which are known in the art. Generallyspeaking, these protocols require each node to listen to the sharedcommunication medium before transmitting. Only if the sharedcommunication medium is available is the node allowed to transmit itsdata. In persistent CSMA, when a node senses that the sharedcommunication medium is not available, the node continually listens tothe shared communication medium and attempts to transmit as soon as themedium becomes available. In non-persistent CSMA, when a node sensesthat the shared communication medium is not available, the node waits anamount of time before attempting to listen to the shared communicationchannel for an opportunity to transmit. Even though a node listens firstbefore transmitting, there still exists a probability for collisions.This is because when the medium is available, two or more nodes candetect the availability and decide that they are going to transmit data.Various techniques have been developed to handle such collisions.

Yet another category of multiple access systems utilizes contention freeprotocols. Here, each node can reserve the shared communication mediumin order to transmit data. The node can transmit data without collidingwith transmissions from other nodes. This is because the sharedcommunication medium is reserved, for a particular time duration forexample, for the node's transmission and not for any other transmission.A significant advantage of contention free protocols is that thecommunication medium is not taken up by unsuccessful transmissions thatcollide with one another and resulting re-transmission attempts. Thiscan lead to a more efficient use of the shared communication medium,especially as the number of nodes and number of data transmissionsincrease.

However, contention free protocols require a reservation process thatallows nodes to reserve use of the shared communication medium. Makingsuch reservations also requires communications. If the reservationprocess itself occupies too much of the shared communication medium,performance of the system can be negatively impacted. Thus, to take fulladvantage of the benefits of contention free multiple access, moreefficient systems for reservation of the shared communication medium areneeded.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to methods and apparatuses for conductingcommunications over a shared communication medium involving a pluralityof nodes, involving (a) sending a request from a first node in theplurality of nodes, the shared communication medium organized to includea request signal space and a scheduled transmission signal space, therequest signal space including a plurality of request segments eachhaving a different location within the request signal space, thescheduled transmission signal space including a plurality of scheduledtransmission segments each having a different location within thescheduled transmission signal space, the request sent in a requestsegment that is one of the plurality of request segments, (b) obtainingan assignment, in a first tier of assignments, associating the requestwith a scheduled transmission segment selected from the plurality ofscheduled transmission segments, the assignment taking into accountlocation of the request within the request signal space, (c) from thefirst node, sending a data transmission in the scheduled transmissionsegment associated with the request in accordance with the assignment,and (d) wherein the plurality of scheduled transmission segmentsincludes at least one default use segment, the at least one default usesegment being available for use by a default entity if the at least onedefault use segment is not assigned in the first tier of assignments.

In one embodiment, an assignment in a second tier of assignmentsassociates the at least one default use segment with the default entity.The assignment in the second tier of assignments may remain in effectfor a specified time duration. The assignment in the second tier ofassignments may define a block of scheduled transmission segments as theat least one default use segment. The default entity may be a defaultnode selected from the plurality of nodes. The default node may utilizethe at least one default use segment for file transfers. The defaultnode may utilize the at least one default use segment for low prioritydata flows.

In one embodiment, the default entity is a group of default nodesselected from the plurality of nodes. The group of default nodes mayshare the at least one default use segment on a contention access basis.The default entity may be selected from a plurality of default entitiesincluding at least one default node and at least one group of defaultnodes.

The present invention further relates to methods and apparatuses forconducting communications over a shared communication medium involving aplurality of nodes including a first node and a second node, involving(a) at the second node, receiving a request from the first node, theshared communication medium organized to include a request signal spaceand a scheduled transmission signal space, the request signal spaceincluding a plurality of request segments each having a differentlocation within the request signal space, the scheduled transmissionsignal space including a plurality of scheduled transmission segmentseach having a different location within the scheduled transmissionsignal space, the request sent in a request segment that is one of theplurality of request segments, (b) at the second node, making anassignment, in a first tier of assignments, associating the request witha scheduled transmission segment selected from the plurality ofscheduled transmission segments, the assignment taking into accountlocation of the request within the request signal space, and sending acorresponding assignment message, (c) at the second node, receiving adata transmission from the first node in the scheduled transmissionsegment associated with the request, and (d) wherein the plurality ofscheduled transmission segments includes at least one default usesegment, the at least one default use segment being available for use bya default entity if the at least one default use segment is not assignedin the first tier of assignments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 presents a simplified network including a scheduler node 102 anda plurality of access nodes 104, 106, 108, and 110 utilizing a sharedcommunication medium, according to one embodiment of the invention.

FIG. 2 depicts a time-division multiplexing scheme as applied to afrequency channel having a bandwidth of 32 Hz over a duration of 1second.

FIG. 3 depicts a frequency division multiplexing scheme as applied to afrequency channel having a bandwidth of 32 Hz over a duration of 1second.

FIG. 4 depicts a wavelet-division multiplexing scheme as applied to afrequency channel having a bandwidth of 32 Hz over a duration of 1second.

FIG. 5 is an illustrative signal diagram showing time divisionmultiplexing (TDM) as utilized to partition the request signal space andthe scheduled transmission signal space, with TDM request segments andscheduled transmission segments, according to an embodiment of theinvention.

FIG. 6 is an illustrative signal diagram showing a two-service TDMrequest and scheduled transmission scheme, in accordance with anembodiment of the invention.

FIG. 7 is an illustrative signal diagram showing orthogonal frequencydivision multiplexing (OFDM) as utilized to partition the request signalspace and the scheduled transmission signal space, with OFDM/TDM requestsegments and scheduled transmission segments, according to an embodimentof the invention.

FIG. 8 is an illustrative signal diagram showing another example oforthogonal frequency division multiplexing (OFDM) as utilized topartition the request signal space and the scheduled transmission signalspace, with OFDM/TDM request segments and scheduled transmissionsegments, according to an embodiment of the invention.

FIG. 9 is an illustrative signal diagram showing frequency divisionmultiplexing (FDM) as utilized to partition the request signal space andthe scheduled transmission signal space, with FDM/TDM request segmentsand scheduled transmission segments, according to an embodiment of theinvention.

FIG. 10 is an illustrative signal diagram showing time divisionmultiplexing (TDM) as utilized to partition the request signal space andthe scheduled transmission signal space, with synchronous CDM requestsegments, according to an embodiment of the invention.

FIG. 11 is an illustrative signal diagram showing time divisionmultiplexing (TDM) as utilized to partition the request signal space andthe scheduled transmission signal space, with quasi-synchronous CDMrequest segments, according to an embodiment of the invention.

FIG. 12 is an illustrative signal diagram showing code divisionmultiplexing (CDM) as utilized to partition the request signal space andthe scheduled transmission signal space, with CDM request segments andCDM scheduled transmission segments, according to an embodiment of theinvention.

FIG. 13 presents a more detailed example of a system with a contentioncode request channel, according to an embodiment of the invention.

FIG. 14 presents a more detailed example of a system with a polled coderequest channel, according to an embodiment of the invention.

FIG. 15 is a plot of expected envelop detector performance underdifferent noise levels, according to an embodiment of the invention.

FIG. 16 depicts use of piggyback requests for request collisiondetection and missed request processing, according to an embodiment ofthe invention.

FIG. 17 presents a simplified network operating under a “no schedulermode,” according to one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to communications conducted over a sharedcommunications medium involving a plurality of nodes. The invention isspecifically related to techniques employed for assigning opportunitiesfor transmission based on requests.

FIG. 1 presents a simplified network including a scheduler node 102 anda plurality of access nodes 104, 106, 108, and 110 utilizing a sharedcommunication medium, according to one embodiment of the invention. Thisconfiguration corresponds to a mode of operation referred to here as“scheduler mode,” which is described as an illustrative example.

Referring to FIG. 1, scheduler node 102 serves to control usage of theshared communication medium by access nodes 104, 106, 108, and 110. Theshared communication medium can represent any communication medium thatmay be utilized by more than one node. For example, the sharedcommunication medium can represent signal space in one or more satellitechannels. Thus, the access nodes and the scheduler node may be part of asatellite network. As another example, the shared communication mediumcan represent signal space in one or more wireless terrestrial channels.Thus, the access nodes and the scheduler node may be part of aterrestrial wireless network. As yet another example, the sharedcommunication medium can represent signal space in one or more wiredchannels. Thus, the access nodes and scheduler node may be part of awired network.

Furthermore, embodiments of the present invention may be implemented indifferent network topologies that involve a shared communication medium.These may include star topologies, mesh topologies, bus topologies, andothers.

In the present embodiment of the invention, scheduler node 102 providescontrol over access of the shared communication medium by access nodes104, 106, 108, and 110. In order to transmit data over the sharedcommunication medium, an access node such as access nodes 104, 106, 108,and 110 first send a request to scheduler node 102. In response,scheduler node 102 assigns an opportunity for data transmission to theaccess node. Scheduler node 102 sends an assignment message associatedwith the assignment to the access nodes. Upon receiving the assignment,the access node that made the request can transmit data in the assignedtransmission opportunity. This general scheme of request, assignment,and transmission is used in various embodiments of the invention.However, as noted before, other embodiments of the invention may involvevariations and different operations.

Symbols

Generally speaking, a basic unit of data transmission is referred tohere as a “symbol.” A symbol can be defined to have one out of a numberof possible values. For example, a binary symbol may have one of twopossible values, such as “0” and “1.” Thus, a sequence of N binarysymbols may convey 2^(N) possible messages. More generally speaking, anM-ary symbol may have M possible values. Thus, a sequence of N M-arysymbols may convey M^(N) possible messages.

The concept of symbol and the methods by which a symbol can assumevalues is quite general. In many applications, a symbol is associatedwith a defined baseband pulse shape which is up-converted to a carrierfrequency with a particular phase relationship to the carrier and with aparticular amplitude. The amplitude and/or phase of the symbol is knownas the modulation and carries the information of a symbol. The set ofpermissible modulation points defined in the amplitude and phase planeis known as the modulation constellation. The amount of information thata symbol may convey is related to the number of discrete points of theconstellation. 16-QAM is an example of amplitude-phase constellationwhich allows transmission of up to 4 bits of information per symbol. Insome applications, only the phase is used for modulation. Quadra-phaseshift keying (QPSK) is an example of pure phase modulation which allowstransmission of up to 2 bits of information per symbol. In otherapplications, the symbol waveform may be defined such that symbol phasemay either not exist or be difficult to receive accurately, in whichcase pure amplitude modulation can be used. One example of binaryamplitude modulation is on-off amplitude-shift keying modulation whichallows transmission of up to 1 bit of information per symbol.

Each symbol may occupy a particular portion of the relevant signalspace. Specifically, each symbol may be said to occupy a certain amount“time-bandwidth product.” Here, an amount of time-bandwidth product is ascalar quantity that may be measured in units of Hz-seconds and does notnecessarily dictate how the signal is distributed within the signalspace. In theory, symbols cannot be strictly limited in both time andfrequency. It is customary, however, to define the time-bandwidthproduct of a signal to be the time-bandwidth product of the region inwhich the preponderance of signal energy resides. Since precisedefinitions of time-bandwidth product vary somewhat throughout theliterature, figures showing symbol boundaries in time-frequency spaceshould only be considered as approximate representations.

Just as a simple example, a signal spanning a bandwidth of 1 Hz andlasting a duration of 1 second may have a time-bandwidth product of 1Hz-second. A signal spanning a bandwidth of 0.5 Hz and lasting aduration of 2 seconds may also have a time-bandwidth product of 1Hz-second. Similarly, a signal spanning a bandwidth of 0.1 Hz andlasting a duration of 10 seconds may also have a time-bandwidth productof 1 Hz-second. These examples do not assume any multiplexing of thesignal space, which is discussed separately below. Also, the particularvalues used in these example and other examples described herein are forillustrative purpose only. Different values may be used in actualsystems.

The measurement of a symbol in terms of an amount of time-bandwidthproduct is also applicable when different signal space multiplexingtechniques are employed. Such techniques may include time-divisionmultiplexing, frequency-division multiplexing, wavelet-divisionmultiplexing, code-division multiplexing, and/or others. In each of thefollowing four examples, a symbol occupies a time-bandwidth product of 1Hz-second, even though different signal space multiplexing techniquesare used.

In a first example, FIG. 2 depicts a time-division multiplexing schemeas applied to a frequency channel having a bandwidth of 32 Hz over aduration of 1 second. The channel is divided into 32 time slots, eachhaving a duration of 1/32 second. A symbol may be transmitted in each1/32-second time slot over the bandwidth of 32 Hz. In this example, eachsymbol has a time-bandwidth product of 1 Hz-second.

In a second example, FIG. 3 depicts a frequency division multiplexingscheme as applied to a frequency channel having a bandwidth of 32 Hzover a duration of 1 second. The channel is divided into 32 differentfrequency sub-channels each having a bandwidth of 1 Hz. A symbol may betransmitted in each 1 Hz frequency sub-channel over the duration of 1second. In this example, each symbol also has a time-bandwidth productof 1 Hz-second.

In a third example, FIG. 4 depicts a wavelet-division multiplexingscheme as applied to a frequency channel having a bandwidth of 32 Hzover a duration of 1 second. The channel is divided into 32 differenttime and frequency symbol segments. 2 symbol segments have a bandwidthof 1 Hz with a duration of 1 second, 2 other symbol segments have abandwidth of 2 Hz with a duration of ½ second, 4 other symbol segmentshave a bandwidth of 4 Hz with a duration of ¼ second, 8 other symbolsegments have a bandwidth of 8 Hz with a duration of ⅛ second, and 16additional symbol segments have a bandwidth of 16 Hz with a duration of1/16 second, In this example, each symbol has a time-frequency productof 1 Hz-second, as well.

In a fourth example, a code-division multiplexing scheme is applied to afrequency channel having a bandwidth of 32 Hz over a duration of 1second. For this example, it is assumed that there are 32 differentpossible orthogonal code words, each comprising a unique 32-chip binarypattern. Each code word represents a unique “code channel.” To send asymbol on a particular code channel, the symbol value is used tomodulate the code word associated with the code channel, and theresulting signal is sent. In the case of binary phase shift keying(BPSK) symbols, for instance, a symbol having a value of “1” may be sentby simply sending the code word, and a symbol having a value of “0” maybe sent by sending the inverted version (180-degree phase shift) of thecode word. The 32 symbols sent using 32 different “code channels” arenon-interfering, and as a group, they occupy a common 32 Hz by 1 secondportion of the time-frequency space. In this example, each symbol has aneffective time-frequency product of 1 Hz-second.

Symbol-Level Request

Referring back to FIG. 1, a symbol-level request may be sent from anaccess node such as access nodes 104, 106, 108, and 110 according to anembodiment of the present invention. Here, a symbol-level request refersto a request that can be sent in the form of a transmission signalhaving a time-bandwidth product comparable to that of a symbol. Forexample, a symbol-level request may occupy exactly one symbol. Thus, aprotocol message comprising a large number of symbols, representing aheader and a data payload that must be processed and interpreted, wouldnot be considered a symbol-level request.

The novel use of a symbol-level request according to embodiments of thepresent invention allows for highly efficient utilization of theavailable signal space. Because of its compact size, a symbol-levelrequest may not have sufficient capacity to carry a significant datapayload. However, according to various embodiments of the invention,information may be conveyed in the choice of the location within therequest signal space in which the symbol-level request is transmitted.Thus, the existence of a symbol-level request in the request signalspace, as well as the location where the symbol-level request exists inthe request signal space, can convey important information that is usedto facilitate the assignment of transmission opportunities within theshared communication medium.

Request Signal Space and Scheduled Transmission Signal Space

According to various embodiments of the invention, the sharedcommunication medium utilized by access nodes 104, 106, 108, and 110 maybe organized into a request signal space and a scheduled transmissionsignal space. Just as an example, the shared communication medium may beimplemented as a satellite “return-link” that allows signals to be sentfrom access nodes 104, 106, 108, and 110 to scheduler node 102.

The request signal space may be used by access nodes 104, 106, 108, and110 to send requests—e.g., symbol-level requests—to requestopportunities for the scheduled transmission of data. Specifically, therequest signal space may be organized into a plurality of requestsegments. Each request segment generally refers to a portion of requestsignal space that may be used for sending a request.

The scheduled transmission signal space may be used by access nodes 104,106, 108, and 110 to transmit data, once requests for transmission havebeen granted. The scheduled transmission signal space may be organizedinto a plurality of scheduled transmission segments. Each scheduledtransmission segment generally refers to a portion of scheduledtransmission signal space that may be used for sending a datatransmission.

In accordance with the invention, the request signal space, as well asthe scheduled transmission signal space, may be organized based onvarious multiplexing techniques. Thus, the plurality of request segmentsin the request signal space may represent allotments defined based onone or more types of multiplexing techniques applied to the requestsignal space. As mentioned previously, these may include time-divisionmultiplexing, frequency-division multiplexing, wavelet-divisionmultiplexing, code-division multiplexing, and/or other multiplexingtechniques. Similarly, the plurality of scheduled transmission segmentsin the scheduled transmission space may represent allotments definedbased on one or more types of multiplexing techniques applied to thescheduled transmission signal space.

As such, each request segment may have a different “location” within therequest signal space. For example, if a request signal space isorganized according to a time-division multiplexing technique, eachrequest segment may comprise a different time slot in the request signalspace. Here, each particular request segment is said to correspond to adifferent location (in time) in the request signal space. The sameconcept can be applied to a request signal space organized according toa frequency-division multiplexing technique. In such a case, eachrequest segment may comprise a particular frequency sub-channel and besaid to correspond to a different location (in frequency) in the requestsignal space. The same concept can be applied to a request signal spaceorganized according to a code-division multiplexing technique. In such acase, each request segment may comprise a particular code word and besaid to correspond to a different location (in code space) in therequest signal space. Similarly, the concept can be applied to a requestsignal space organized according to a combination of differentmultiplexing techniques, such as a combination of time-divisionmultiplexing and frequency-division multiplexing techniques. In thisparticular example, each request segment may comprises a particular timeslot in a particular frequency sub-channel and be said to correspond toa different location (in time and frequency) in the request signalspace.

Also, the separation between the request signal space and the scheduledtransmission signal space may be based on different multiplexingtechniques. In one embodiment, time-division multiplexing is employed.For example, the request signal space and the scheduled transmissionsignal space may be defined over different time slots and a commonfrequency range. In another embodiment, frequency-division multiplexingis employed. For example, the request signal space and the scheduledtransmission signal space may be defined over a common time duration anddifferent frequency ranges. In yet another embodiment, code-divisionmultiplexing is employed. For example, the request signal space and thescheduled transmission signal space may be defined over a common timeduration and a common frequency range, but using different code words.

Feedback Signal Space

In addition, a feedback signal space may be utilized for sending theassignment messages from scheduler node 102 to access nodes 104, 106,108, and 110. In the present embodiment of the invention, the feedbackspace is not a part of the shared communication medium. Continuing witha satellite system example, the feedback signal space may be implementedas a satellite “forward-link” that allows signals to be sent fromscheduler node 102 to access nodes 104, 106, 108, and 110. Thissatellite “forward-link” may be separate from the “return-link”mentioned previously.

The present invention broadly covers different combinations ofmultiplexing techniques as applied to the request signal space and/orthe scheduled transmission signal space. In figures discussed below, anumber of examples of such multiplexing combinations are presented. Thevarious combinations of multiplexing techniques described below arepresented for illustrative purposes and are not intended to restrict thescope of the invention. In some examples, a feedback signal space isalso explicitly shown along with the request signal space and thescheduled transmission signal space.

In the figures below, only a representative portion of the relevantsignal space is shown. For example, if four frames of signals are shown,it should be understood that more frames may be used even though theyare not explicitly illustrated. Also, the particular proportions of thevarious signal space designs are provided as mere examples.

TDM Request Signal Space and Scheduled Transmission Signal SpacePartitioning with TDM Request Segments and Scheduled TransmissionSegments

FIG. 5 is an illustrative signal diagram showing time divisionmultiplexing (TDM) as utilized to partition the request signal space andthe scheduled transmission signal space, with TDM request segments andscheduled transmission segments, according to an embodiment of theinvention. The figure shows a representation of shared communicationmedium 200 that includes a request signal space and a scheduledtransmission signal space. Separately, the figure shows a feedbacksignal space 250.

In this particular embodiment of the invention, the shared communicationmedium 200 is organized as one continuous sequence of TDM time slots.For example, the shared communication medium 200 may comprise aparticular frequency channel. Each TDM time slot occupies the entirebandwidth of the frequency channel, but only for a specific timeduration. Here, the TDM time slots are shown as being organized into“frames,” such as Frame 0, Frame 1, Frame 2, and Frame 3. For ease ofillustration, FIG. 5 presents the TDM time slots in multiple columns,instead of one continuous column. Nevertheless, it should be understoodthat the TDM time slots represent a single sequence of time slots thatare sequentially transmitted.

For example, FIG. 5 shows that Frame 0 includes 512 TDM time slots.These 512 TDM time slots are shown as being arranged in a rectangulargrid having 16 columns (Column 0 through Column 15) and 32 rows (Row 0through Row 31). The sequence of TDM time slots is arranged in time asfollows. Slots 0 through 31 of Column 1, followed by Slots 0 through 31of Column 2, followed by Slots 0 through 31 of Column 3, and so on. Inthis manner, the entire sequence of 512 time slots in Frame 0 isarranged sequentially in time. Frame 1 is structured similarly andfollows Frame 0. That is, the last time slot of Frame 0 is followed bythe first slot of Frame 1. Frame 2 is structured similarly and followsFrame 1. Frame 3 is structure similarly and follows Frame 2, and so on.Thus, the entire sequence of TDM time slots contained in all the frames,including Frames 0, 1, 2, and 3, is arranged sequentially in time.

In the particular grid representation shown in FIG. 5, time can be seenas proceeding down each column of TDM time slots. Hence the directiondown each column, across multiple rows, is labeled as “Fast Row Time.”Only after proceeding through all the TDM time slots in a column can thenext column be started. Thus, it takes longer to proceed from one columnto the next. Hence the direction across multiple columns is labeled as“Slow Column Time.”

In FIG. 5, the request signal space and scheduled transmission signalspace are defined on the basis of these TDM time slots. Thus, in thisexample, the request signal space and the scheduled transmission signalspace are separated using TDM multiplexing. Here, each frame includes anumber of request segments and a single scheduled transmission segment.In Frame 0, for example, the first 32 time slots are considered 32request segments (Column 0). The next 480 time slots are considered onescheduled transmission segment, made up of 480 symbols (Columns 1through 15). Other frames such as Frames 0, 1, 2, and 3 are structuredin a similar manner.

FIG. 5 shows the following types of signals allotments: (1) UnoccupiedRequest Slot, (2) Occupied Request Slot, and (3) Scheduled TransmissionData Symbol. In this example, only some of the available requestsegments are occupied by actual requests sent from one or more accessnodes. Thus, some request segments are shown as unoccupied requestslots, and others are shown as occupied request slots. Each scheduledtransmission segment is shown as including a number of symbols, referredto as schedule transmission data symbols.

According to an embodiment of the invention, when an access node such asnodes 104, 106, 108, and 110 in FIG. 1 needs to request a scheduledtransmission, it sends out a request in one of the request segments.Here, it is assumed that a TDM system is implemented in which all of thenodes are time-synchronized, such that every node has the capability tosend signals in the appropriate time slots. Of course in practice,signals sent from various nodes may not arrive in their respective timeslots with perfect timing accuracy. The TDM system may be designed tohandle such imperfections, up to certain tolerances.

In one example, the request signals shown in FIG. 5 may be sent asfollows. Node 104 may send a request in Slot 7 of the request signalspace (Column 0) in Frame 0. Node 106 may send a request in Slot 2 ofthe request signal space (Column 0) in Frame 1. Node 108 may send arequest in Slot 26 of the request signal space (Column 0) in Frame 1.Finally, node 110 may send a request in Slot 18 of the request signalspace (Column 0) in Frame 3. Again, access nodes 104, 106, 108, and 110are shown in FIG. 1. Of course, an access node may also send multiplerequests, sometimes in the same frame. Thus, in an alternative example,all four of the requests shown in FIG. 5 may be sent from node 104. Thatis, node 104 may send the request in Slot 7 of the request signal space(Column 0) in Frame 0, the requests in Slot 2 and Slot 26 of the requestsignal space (Column 0) in Frame 1, and the request in Slot 18 of therequest signal space (Column 0) in Frame 3.

According to an embodiment of the invention, scheduler node 102 shown inFIG. 1 receives the requests and makes assignments to assign eachrequest to a scheduled transmission segment. Thus, in response to therequests, scheduler 102 send out assignment messages in a feedbacksignal space 250. The assignment messages are broadcast to access nodes104, 106, 108, and 110, to inform the access nodes of the assignmentsmade, so that each access node may correctly send data in the assignedscheduled transmission segment(s).

FIG. 5 depicts an assignment message under a “robust FIFO schedulemode,” in accordance with one embodiment of the invention. In this mode,each assignment message explicitly includes a pair of data: (1) anidentifier for the request and (2) an identifier for the scheduledtransmission segment associated with the request. In other words, thepairing of a request to an associated scheduled transmission segment isdirectly stated in the assignment message. For example, as shown in FIG.5, the first assignment message includes the pair of data “REQ 0:7, SCH13.” This indicates that the request sent in the request segment knownas “REQ 0:7” (the request segment located at Frame 0, Slot 7) has beenassigned the scheduled transmission segment known as “SCH 13” (thescheduled transmission segment located in Frame 13). The rest of theassignment messages follow a similar format. The second assignmentmessage includes the pair of data “REQ 1:2, SCH 14.” The thirdassignment message includes the pair of data “REQ 1:26, SCH 15.” Thefourth assignment message includes the pair of data “REQ 3:18, SCH 16.”

The entire request and assignment process takes place in an anonymousmanner with respect to the identity of the access nodes, according to anembodiment of the invention. Thus, a symbol-level request sent from anaccess node does not explicitly identify the access node. For example,assume that access node 104 sent the symbol-level request in “REQ 0:7”(Slot 7 of the request signal space of Frame 0). This symbol-levelrequest is merely a symbol transmitted at a particular location withinthe request symbol space. The symbol-level request does not explicitlyidentify the access node 104. Similarly, the corresponding assignmentmessage “REQ 0:7, SCH 13” broadcast from scheduler node 102 does notexplicitly identify access node 104 as the intended recipient of theassignment message. Instead, the assignment message merely announcesthat the symbol-level request sent in the “REQ 0:7” slot has beenassigned to the scheduled transmission segment “SCH 13.” All of theaccess nodes 104, 106, 108, and 110 receive the broadcast assignmentmessage. However, only access node 104 accepts the assignment andproceeds to send a data transmission in the scheduled transmissionsegment identified by the assignment. This is possible because eachaccess node keeps track of the location(s) of the symbol-levelrequest(s) it has sent. Access node 104 recognizes the request “REQ 0:7”identified in the assignment as one of its own and thus accepts theassignment. The other access nodes 106, 108, and 110 do not recognizethe request “REQ 0:7” identified in the assignment as one of their ownand thus do not accept the assignment.

In FIG. 5, the feedback signal space 250 is labeled as “Delayed FeedbackGrant Channel.” In this particular embodiment of the invention, anassignment message sent in feedback signal space 250 may be delayed inthe sense that it may not be broadcast until some time after (perhapsmultiple frame after) the initial request is made.

Two-Service TDM Request and Scheduled Transmission

FIG. 6 is an illustrative signal diagram showing a two-service TDMrequest and scheduled transmission scheme, in accordance with anembodiment of the invention. The system shown in FIG. 6 is similar tothe system shown in FIG. 5 in many respects. FIG. 6 shows arepresentation of a shared communication medium 300 that includes arequest signal space and a scheduled transmission signal space. TDM isutilized to partition the request signal space and the scheduledtransmission signal space. TDM request segments and scheduledtransmission segments are also shown. Separately, FIG. 6 shows afeedback signal space 350.

Unlike FIG. 5, FIG. 6 introduces a two-service methodology. Thedistinction between different services may be based on one or morefactors, such as the length of scheduled transmission, code rate,modulation method, and/or others. In the example shown in FIG. 6, thedistinction between the two services is based on length of the scheduledtransmission. Thus, an access node can either request a short scheduledtransmission of data (1^(st) service) or request a long scheduledtransmission of data (2^(nd) service). Here, a short scheduledtransmission of data is also referred to as a “short burst,” and a longscheduled transmission of data is also referred to as a “long burst.”

According to an embodiment of the invention, the request segments areorganized into two categories, one for the first service and the otherfor the second service. Similarly, the scheduled transmission segmentsare organized into two categories, short scheduled transmission segmentsfor the first service and long scheduled transmission segments for thesecond service. An access node such as access nodes 104, 106, 108, and110 chooses the service desired by sending a request in a requestsegment of the appropriate category. To request a short scheduledtransmission of data (1^(st) service), an access node simply sends arequest in one of the 1^(st) service request segments. To request a longscheduled transmission of data (2^(nd) service), the access node simplysends a request in one of the 2^(nd) service request segments.

FIG. 6 shows the following types of signals allotments: (1) Empty ShortBurst Request Slot, (2) Empty Long Burst Request Slot, (3) OccupiedRequest Slot, and (4) Scheduled Transmission Data Symbol. The requestsegments comprise short burst request slots and long burst requestslots. As shown in this example, only some of the available requestsegments are occupied by actual requests sent from one or more accessnodes. Thus, some request segments are shown as unoccupied requestslots, and others are shown as occupied request slots. Each scheduledtransmission segment is shown as including a number of symbols, referredto as schedule transmission data symbols.

In accordance with an embodiment of the invention, the data framestructure may be used to organize the two categories of request segmentsand scheduled transmission segments. In this figure, even numberedframes such as Frames 0, 2, 4, etc., contain request segments andscheduled transmission segments for short scheduled transmissions ofdata (1^(st) service). Odd numbered frames such as Frames 1, 3, 5, etc.,contain request segments and scheduled transmission segments for longscheduled transmission of data (2^(nd) service).

FIG. 6 presents a number of illustrative requests for the two servicetypes, as sent from one or more access nodes such as 104, 106, 108, and110. A short burst request is sent in Slot 7 of the request signal spacein Frame 0. Two long burst requests are sent in Slots 2 and 26 of therequest signal space in Frame 1. A short burst request is sent in Slot22 of the request signal space in Frame 2. A long burst request is sentin Slot 8 of the request signal space in Frame 3. Finally, a short burstrequest is sent in Slot 19 of the request signal space in Frame 4.

In response, scheduler node 102 broadcasts assignment messages infeedback signal space 350. As shown in FIG. 6, the first assignmentmessage “REQ 0:07, SHORT 12” indicates that the request sent in Slot 7of the request signal space in Frame 0 has been assigned to the shortscheduled transmission segment in Frame 12. The second assignmentmessage “REQ 1:02, LONG 17” indicates that the request sent in Slot 2 ofthe request signal space in Frame 1 has been assigned to the longscheduled transmission segment in Frame 17. The third assignment message“REQ 1:26, LONG 19” indicates that the request sent in Slot 26 of therequest signal space in Frame 1 has been assigned to the long scheduledtransmission segment in Frame 19. The fourth assignment message “REQ2:22, SHORT 14” indicates that the request sent in Slot 22 of therequest signal space in Frame 1 has been assigned to the short scheduledtransmission segment in Frame 14. The fifth assignment message “REQ3:08, LONG 21” indicates that the request sent in Slot 8 of the requestsignal space in Frame 3 has been assigned to the long scheduledtransmission segment in Frame 21. The fifth assignment message “REQ4:19, SHORT 16” indicates that the request sent in Slot 19 of therequest signal space in Frame 4 has been assigned to the short scheduledtransmission segment in Frame 16.

The assignment messages are broadcast to access nodes 104, 106, 108, and110, to inform the access nodes of the assignments made, so that eachaccess node may correctly send data in the assigned scheduledtransmission segment(s). While a two-service example is presented inFIG. 6, systems having more than two types of service are within thescope of the present invention. Such systems can be implemented, forexample, by adopting a signal space design similar to that shown in FIG.6 and scaling up the number of categories of request segments andscheduled transmission segments.

OFDM/TDM Request and Scheduled Transmission

FIG. 7 is an illustrative signal diagram showing orthogonal frequencydivision multiplexing (OFDM) as utilized to partition the request signalspace and the scheduled transmission signal space, with OFDM/TDM requestsegments and scheduled transmission segments, according to an embodimentof the invention. The figure shows a representation of a sharedcommunication medium 400 based on an OFDM structure that includes arequest signal space and a scheduled transmission signal space.Separately, the figure shows a feedback signal space 450. In thisparticular embodiment of the invention, the shared communication medium400 is organized into 34 different OFDM frequency channels, labeled assubcarriers −1 through −17 and 1 through 17. Each of these 34 differentOFDM frequency channels is further organized into TDM time slots. The DCchannel, labeled as subcarrier 0, is left unused.

According to the present embodiment of the invention, OFDM frequencychannels provide a natural separation between the request signal spaceand the scheduled transmission signal space. As shown in FIG. 7, therequest signal space comprises OFDM frequency channels −1 and 1. Thescheduled transmission signal space comprises OFDM frequency channels −2through −17 and 2 through 17.

In this embodiment, each request segment comprises a TDM time slot inone of the OFDM frequency channels −1 and 1. Each scheduled transmissionsegment comprises a block of 512 data symbols, spanning 16 OFDMfrequency channels and 32 TDM time slots.

FIG. 7 shows four such scheduled transmission segments. The firstscheduled transmission segment spans OFDM frequency channels −2 through−17 and TDM time slots 0 through 31. The second scheduled transmissionsegment spans OFDM frequency channels 2 through 17 and TDM time slots 0through 31. The third scheduled transmission segment spans OFDMfrequency channels −2 through −17 and TDM time slots 32 through 63. Thefourth scheduled transmission segment spans OFDM frequency channels 2through 17 and TDM time slots 32 through 63. Thus, FIG. 7 presentsrequest segments based on OFDM and TDM, as well as scheduledtransmission segments based on OFDM and TDM.

FIG. 7 shows the following types of signals allotments: (1) UnoccupiedRequest Slot, (2) Occupied Request Slot, and (3) Scheduled TransmissionData Burst. In this example, only some of the available request segmentsare occupied by actual requests sent from one or more access nodes.Thus, some request segments are shown as unoccupied request slots, andothers are shown as occupied request slots. Each scheduled transmissionsegment comprises a block of 512 data symbols referred to here as a databurst.

FIG. 7 presents a number of illustrative requests, as sent from one ormore access nodes such as 104, 106, 108, and 110. A request is sent inSlot 8 in OFDM frequency channel −1. Another request is sent in Slot 18in OFDM frequency channel 1. Another request is sent in Slot 22 in OFDMfrequency channel 1. Yet another request is sent in Slot 57 in OFDMfrequency channel 1.

In response, scheduler node 102 broadcasts assignment messages infeedback signal space 450. As shown in FIG. 7, the first assignmentmessage “REQ −8, SCH +21” indicates that the request sent in Slot 8 inOFDM frequency channel −1 has been assigned to the scheduledtransmission segment +21. In this example, a scheduled transmissionsegment is identified by a sign (positive or negative) and a sequencenumber. For instance, the scheduled transmission segment +21 has apositive sign (+), which indicates that this scheduled transmissionsegment is located on the positive side of the OFDM frequency channels(spanning the 16 positive OFDM frequency channels). The sequence number21 indicates that this scheduled transmission segment is the 21^(st) onein a sequence of scheduled transmissions.

The rest of the assignment messages can be interpreted in a similar way.The second assignment message “REQ +18, SCH −22” indicates that therequest segment in Slot 18 in OFDM frequency channel 1 has been assignedto the scheduled transmission segment −22. The third assignment message“REQ +22, SCH +22” indicates that the request segment in Slot 22 in OFDMfrequency channel 1 has been assigned to the schedule transmissionsegment +22. The fourth assignment message “REQ +57, SCH −23” indicatesthat the request segment in Slot 57 in OFDM frequency channel 1 has beenassigned to the scheduled transmission segment −23.

The assignment messages are broadcast to access nodes 104, 106, 108, and110, to inform the access nodes of the assignments made, so that eachaccess node may correctly send data in the assigned scheduledtransmission segment(s).

FIG. 8 is an illustrative signal diagram showing another example oforthogonal frequency division multiplexing (OFDM) as utilized topartition the request signal space and the scheduled transmission signalspace, with OFDM/TDM request segments and scheduled transmissionsegments, according to an embodiment of the invention. The figure showsa representation of a shared communication medium 500 based on an OFDMstructure that includes a request signal space and a scheduledtransmission signal space. Separately, the figure shows a feedbacksignal space 550. The shared communication medium 500 is once againorganized into 34 different OFDM frequency channels, labeled assubcarriers −1 through −17 and 1 through 17. Each of these 34 differentOFDM frequency channels is further organized into TDM time slots. The DCchannel, labeled as subcarrier 0, is left unused.

This example demonstrates that the request signal space and scheduledtransmission signal space can be defined in a different way. Again, OFDMfrequency channels are used to provide the separation between requestsignal space and scheduled transmission signal space. However, adifferent pair of OFDM frequency channels is used here to implement therequest signal space. Here, the request signal space comprises OFDMfrequency channels −17 and 17. The scheduled transmission signal spacecomprises OFDM frequency channels −1 through −16 and 1 through 16.

This example also demonstrates that the scheduled transmission segmentscan be defined as utilizing a different allocation of OFDM frequencychannels and TDM time slots. Here, each scheduled transmission segmentstill comprises a block of 512 data symbols. However, the 512 datasymbols span 32 OFDM frequency channels and 16 TDM time slots. FIG. 8shows four such scheduled transmission segments. The first scheduledtransmission segment spans OFDM frequency channels −2 through −17 and 2through 17 and TDM time slots 0 through 15. The second scheduledtransmission segment spans OFDM frequency channels −2 through −17 and 2through 17 and TDM time slots 16 through 31. The third scheduledtransmission segment spans OFDM frequency channels −2 through −17 and 2through 17 and TDM time slots 32 through 47. The fourth scheduledtransmission segment spans OFDM frequency channels −2 through −17 and 2through 17 and TDM time slots 48 through 63.

FIG. 8 shows the following types of signals allotments: (1) UnoccupiedRequest Slot, (2) Occupied Request Slot, and (3) Scheduled TransmissionData Burst. In this example, only some of the available request segmentsare occupied by actual requests sent from one or more access nodes.Thus, some request segments are shown as unoccupied request slots, andothers are shown as occupied request slots. Each scheduled transmissionsegment comprises a block of 512 data symbols referred to here as a databurst.

FIG. 8 presents a number of illustrative requests, as sent from one ormore access nodes such as 104, 106, 108, and 110. A request is sent inSlot 4 in OFDM frequency channel −17. Another request is sent in Slot 8in OFDM frequency channel −17. Another request is sent in Slot 22 inOFDM frequency channel 17. Another request is sent in Slot 33 in OFDMfrequency channel −17. Yet another request is sent in Slot 57 in OFDMfrequency channel 17.

In response, scheduler node 102 broadcasts assignment messages infeedback signal space 550. As shown in FIG. 8, the first assignmentmessage “REQ −4, SCH 11” indicates that the request sent in Slot 4 inOFDM frequency channel −17 has been assigned to the scheduledtransmission segment 11. In this example, a scheduled transmissionsegment is identified by a sequence number. No sign is used because eachscheduled transmission segment spans all OFDM frequency channels,including all positive and negative frequency channels. The sequencenumber 11 indicates that this scheduled transmission segment is the11^(th) one in a sequence of scheduled transmissions.

The rest of the assignment messages can be interpreted in a similar way.The second assignment message “REQ −8, SCH 12” indicates that therequest segment in Slot 8 in OFDM frequency channel −17 has beenassigned to the scheduled transmission segment 12. The third assignmentmessage “REQ +22, SCH 13” indicates that the request segment in Slot 22in OFDM frequency channel 17 has been assigned to the scheduletransmission segment 13. The fourth assignment message “REQ −33, SCH 14”indicates that the request segment in Slot 33 in OFDM frequency channel−17 has been assigned to the scheduled transmission segment 14. Thefifth assignment message “REQ +57, SCH 15” indicates that the requestsegment in Slot 57 in OFDM frequency channel 17 has been assigned to thescheduled transmission segment 15. The assignment messages are broadcastto access nodes 104, 106, 108, and 110, to inform the access nodes ofthe assignments made, so that each access node may correctly send datain the assigned scheduled transmission segment(s).

FDM Request and Scheduled Transmission

FIG. 9 is an illustrative signal diagram showing frequency divisionmultiplexing (FDM) as utilized to partition the request signal space andthe scheduled transmission signal space, with FDM/TDM request segmentsand scheduled transmission segments, according to an embodiment of theinvention. The figure shows a representation of a shared communicationmedium 600 based on an FDM structure that includes a request signalspace and a scheduled transmission signal space. The request signalspace comprises a narrow FDM frequency channel, labeled as REQ Carrier.The scheduled transmission signal space comprises two wide FDM frequencychannels, labeled as SCH Carrier 1 and SCH Carrier 2. A feedback signalspace is not explicitly shown in this figure but may also be implementedin a manner similar to that described with respect to previous figures.

This example demonstrates that the request signal space and thescheduled transmission signal space may have very different symbolstructures. In the request signal space, a symbol is transmitted overthe narrow REQ Carrier channel over a longer time duration. These arelabeled as REQ Symbol Slots 0, 1, 2, 3, etc. in the figure. By contrast,in the schedule transmission signal space, a symbol is transmitted overone of the two wide SCH Carrier channels over a shorter time duration.These are labeled as Data Burst Symbol 0, 1, 2, 3, . . . , 63, etc. inthe figure. Despite this difference in symbol structures, a symboltransmitted in the request signal space may have the same time-bandwidthproduct as a symbol transmitted in the scheduled transmission signalspace, according to an embodiment of the present invention. Thus, FIG. 9presents request segments based on FDM and TDM, as well as scheduledtransmission segments based on FDM and TDM.

FIG. 9 shows the following types of signals allotments: (1) UnoccupiedRequest Slot and (2) Occupied Request Slot. In this example, only someof the available request segments are occupied by actual requests sentfrom one or more access nodes. Thus, some request segments are shown asunoccupied request slots, and others are shown as occupied requestslots. FIG. 9 also shows scheduled transmission segments. Each scheduledtransmission segment comprises a block of N data symbols referred tohere as a data burst. Only a portion of the data burst is shown in thefigure.

FIG. 9 presents an illustrative request as sent from one or more accessnodes such as 104, 106, 108, and 110. The request shown is sent in Slot1 of the REQ Carrier channel. In response, scheduler node 102 broadcastsassignment messages in feedback signal space (not shown). The assignmentmessages are broadcast to access nodes 104, 106, 108, and 110, to informthe access nodes of the assignments made, so that each access node maycorrectly send data in the assigned scheduled transmission segment(s).

Also shown in FIG. 9 are guard zones, specifically frequency guardbands, positioned between various carriers. A first frequency guard bandis positioned between the SCH Carrier 1 channel and REQ Carrier channel.A second frequency guard band is positioned between the REQ Carrierchannel and the SCH Carrier 2 channel. The use of these guard bands canimprove reception and processing of a signal on a particular carrier byproviding separation and reduced interference from neighboring carriers.

TDM Request Signal Space and Scheduled Transmission Signal SpacePartitioning with Synchronous CDM Request Segments

FIG. 10 is an illustrative signal diagram showing time divisionmultiplexing (TDM) as utilized to partition the request signal space andthe scheduled transmission signal space, with synchronous CDM requestsegments, according to an embodiment of the invention. The figure showsa representation of a shared communication medium 700 based on a TDMstructure that includes a request signal space and a scheduledtransmission signal space. A feedback signal space is not explicitlyshown in this figure but may also be implemented in a manner similar tothat described with respect to previous figures.

The structure shown in FIG. 10 is based on sequentially ordered frames.Seventeen such frames are shown in this figure, labeled by frame indices0 through 16. Additional frames may follow. Each frame has a totallength of 456 symbols. This total length is divided between a scheduledtransmission signal space portion having a length of 424 symbols and arequest signal space portion having a length of 32 symbols.

For ease of illustration, the numerous symbols are not individuallyshown in this figure. Instead, boxes representing multiple symbols areshown. In the transmission signal space, each short box represents 8scheduled transmission symbols. In the request signal space, each longbox represents a 32-chip CDMA request interval. Although the signalsegments representing individual chips of any particular CDMA code maybe similar in design to the signal segments representing the scheduledtransmission symbols, the chips of any particular code are linked in aparticular code pattern (e.g., a 32-chip pattern), whereas the scheduledtransmission symbols may be individually modulated. As shown, FIG. 10presents scheduled transmission segments based on TDM and requestsegments based on CDM.

More specifically, each 456-symbol frame supports 1 scheduledtransmission segment and 32 request segments. The 1 scheduledtransmission segment comprises the first 424 symbols of the frame. The32 request segments comprise the 32 possible code words that may betransmitted in the remaining portion of the frame. In other words, theremaining portion of the frame is code division multiplexed andorganized as a 32-chip request interval.

Here, a 32-chip Walsh CDMA code is used. In this code space, there exist32 different possible code words, each having a length of 32 chips.Indices 0 through 31 are used to identify the 32 different possible codewords. FIG. 10 shows the chip-level detail of the 32 code words. Othertypes and lengths of code may be used in accordance with the invention.

One or more of the access nodes 104, 106, 108, and 110 can send one ormore requests (each in the form of one of the 32 possible code words) ina particular request interval. This is illustrated in FIG. 10. In theexample shown, two requests are sent in a request interval. The firstrequest is a signal spread according to code word 13. The second requestis a signal spread according to code word 22. Thus, code divisionmultiplexing allows the request interval to support 32 request segments,i.e., codes slots. As shown in FIG. 10, two of these request segmentsare occupied. The remaining thirty request segments are unoccupied.

In FIG. 10, it is assumed that a TDM system is implemented in which allof the nodes are sufficiently time-synchronized, such that no guard zoneis needed to separate the scheduled transmission symbols and the requestintervals. Even though minor timing offsets may exist between differentCDM request signals, the requests are still received and processed withacceptable performance. As such, FIG. 10 refers to a system having“synchronous” CDM request segments.

In response, scheduler node 102 broadcasts assignment messages infeedback signal space (not shown). The assignment messages are broadcastto access nodes 104, 106, 108, and 110, to inform the access nodes ofthe assignments made, so that each access node may correctly send datain the assigned scheduled transmission segment(s).

Again, the entire request and assignment process takes place in ananonymous manner with respect to the identity of the access nodes. Thus,a symbol-level request sent from an access node does not explicitlyidentify the access node. For example, assume that access node 104 sendsthe symbol-level request comprising code word 13. This symbol-levelrequest is merely a signal transmitted at a particular code locationwithin the request symbol space. The symbol-level request does notexplicitly identify the access node 104.

Similarly, the corresponding assignment message would not explicitlyidentify access node 104 as the intended recipient of the assignmentmessage. Instead, the assignment message merely announces that thesymbol-level request corresponding to code word 13 in Frame 0 has beenassigned to a particular scheduled transmission segment. All of theaccess nodes 104, 106, 108, and 110 receive the broadcast assignmentmessage. However, only access node 104 accepts the assignment andproceeds to send a data transmission in the scheduled transmissionsegment identified by the assignment. This is possible because eachaccess node keeps track of the location(s) in code space of thesymbol-level request(s) it has sent in each frame. Access node 104recognizes the request identified in the assignment as one of its ownand thus accepts the assignment. The other access nodes 106, 108, and110 do not recognize the request identified in the assignment as one oftheir own and thus do not accept the assignment.

TDM Request Signal Space and Scheduled Transmission Signal SpacePartitioning with Quasi-Synchronous CDM Request Segments

FIG. 11 is an illustrative signal diagram showing time divisionmultiplexing (TDM) as utilized to partition the request signal space andthe scheduled transmission signal space, with quasi-synchronous CDMrequest segments, according to an embodiment of the invention. Thefigure shows a representation of a shared communication medium 800 basedon a TDM structure that includes a request signal space and a scheduledtransmission signal space. A feedback signal space is not explicitlyshown in this figure but may also be implemented in a manner similar tothat described with respect to previous figures.

The structure shown in FIG. 11 is similar to that of FIG. 10, exceptthat 8-symbol guard zones are inserted in each frame to separate thescheduled transmission signal space and the request signal space. Thestructure is again based on sequentially ordered frames. Seventeen suchframes are shown in this figure, labeled by frame indices 0 through 16.Additional frames may follow. Each frame has a total length of 472symbols. This total length is divided among four different portions ofthe frame: (1) a guard zone having a length of 8 symbols, (2) ascheduled transmission signal space portion having a length of 424symbols, (3) another guard zone having a length of 8 symbols, and (4) arequest signal space portion having a length of 32 code chips.

For ease of illustration, the numerous symbols are not individuallyshown in this figure. Instead, boxes representing multiple symbols areshown. In the transmission signal space, each short box represents 8scheduled transmission symbols. In the request signal space, each longbox represents a 32-chip CDMA request interval. For the guard zones,each short box represents 8 guard symbols. Thus, FIG. 11 presentsscheduled transmission segments based on TDM and request segments basedon CDM.

A 32-chip, low cross-correlation, quasi-synchronous CDMA code is used inthis embodiment. When two or more of such codes are not synchronouslyaligned in time (e.g., they are mis-aligned by a chip), the crosscorrelation between the codes remains relatively low. Other types andlengths of code may be used in accordance with the invention. One ormore of the access nodes 104, 106, 108, and 110 can send one or morerequests (each in the form of one of the 32 possible code words) in aparticular request interval. In the example shown, two requests are sentin a request interval. The first request is a signal spread according tocode word 13. The second request is a signal spread according to codeword 22. Thus, code division multiplexing allows the request interval tosupport 32 request segments, i.e., codes slots. As shown in FIG. 11, twoof these request segments are occupied. The remaining thirty requestsegments are not unoccupied.

In FIG. 11, it is assumed that a TDM system is implemented in which thenodes are not sufficiently time-synchronized, such that guard zones areneeded to separate the scheduled transmission symbols and the requestintervals. Because of the imperfect time synchronization, differentrequests received may be time-offset from one another. As a result,energy from immediately adjacent scheduled transmission symbols mayinterfere with and degrade the proper reception and processing of therequests. By inserting guard zones, the likelihood of such encroachmentis reduced, thereby allowing better performance when received requests,such as multiple CDM requests received in a particular request interval,are not precisely synchronized in time. As such, FIG. 11 refers to asystem having “quasi-synchronous” CDM request segments.

In response, scheduler node 102 broadcasts assignment messages infeedback signal space (not shown). The assignment messages are broadcastto access nodes 104, 106, 108, and 110, to inform the access nodes ofthe assignments made, so that each access node may correctly send datain the assigned scheduled transmission segment(s).

TDM Request Signal Space and Scheduled Transmission Signal SpacePartitioning with Synchronous CDM Request Segments

FIG. 12 is an illustrative signal diagram showing code divisionmultiplexing (CDM) as utilized to partition the request signal space andthe scheduled transmission signal space, with CDM request segments andCDM scheduled transmission segments, according to an embodiment of theinvention. The figure shows a representation of a shared communicationmedium 900 based on a concatenated CDM structure that utilizes an innercode and an outer code to support a request signal space and a scheduledtransmission signal space. A feedback signal space is not explicitlyshown in this figure but may also be implemented in a manner similar tothat described with respect to previous figures.

The structure shown in FIG. 12 is based on sequentially ordered frames.Two such frames are shown in this figure. Frame #1 comprises the blockof signal space covering “Inner Walsh Code Index” 0 through 31 and“Inner Walsh Code Spread Interval Index” 0 through 15. Frame #2comprises a block of the signal space covering “Inner Walsh Code Index”0 through 31 and “Inner Walsh Code Spread Interval Index” 16 through 31.The frames have similar formats. As such, Frame #1 is described indetail below as an illustrative example.

Frame #1 contains 16 time slots (corresponding to the 16 columns ofFrame #1 shown in FIG. 12). Each time slot represents an inner codeinterval of 32 chips. A detailed view of one such 32-chip inner codeinterval is shown at the bottom of FIG. 12. Here, an “inner code” isemployed to multiplex each time slot into 32 “inner code channels,” onefor each code word. The 32 “inner code channels” are shown as the 32rows of Frame #1 in FIG. 12. Here, a 32-chip Walsh code having 32different possible code words is used.

Thus, instead of transmitting a single symbol in each time slot, up to32 different code symbols can be transmitted in each time slot. Eachcode symbol comprises a particular 32-chip inner code word, modulated bythe data value that is to be sent. For example, the 32-chip code wordfor a particular inner code channel may be [0, 0, 0, . . . , 0]. Here,modulating this inner code word with a data value of “0” results in a32-chip code symbol of [0, 0, 0, . . . 0]. Modulating the same innercode word with a data value of “1” results in a 32-chip code symbol of[1, 1, 1, . . . , 1]. As mentioned above, Frame #1 contains 16 timeslots, and each time slot supports 32 different code symbols(corresponding to 32 inner code channels). Thus, Frame #1 supports 512code symbols.

The multiplexing provided by the “inner code channels” is used topartition Frame #1 into a portion of request signal space and a portionof scheduled transmission signal space. As shown in FIG. 12, Frame #1includes 32 inner code channels labeled as “Inner Walsh Code Index” 0through 32. The first two code channels, 0 and 1, are designated as therequest signal space portion. The remaining thirty code channels, 2through 31, are designated as the scheduled transmission portion. Thus,each frame supports a scheduled transmission segment comprising 480 codesymbols (30 inner code channels×16 time slots). In other words, 480 ofthe 512 code symbols in Frame #1 are use as part of the scheduledtransmission segment.

The request signal space, comprising (1) a first “Outer Request CodeSpread Interval” inner code channel 0 and (2) a second “Outer RequestCode Spread Interval” in code channel 1, is further multiplexed using an“outer” code. The outer code has a code length of 16 chips and spans 16time slots. That is, each chip of the 16-chip outer code spans an entiretime slot. Here, a Walsh code having 16 different possible code wordsand a length of 16 chips is used. As a result, each “Outer Request CodeSpread Interval” is further multiplexed, into 16 “outer code channels.”Each of these outer code channels represents a unique request segment inthe request signal space. Frame #1 has two such “Outer Request CodeSpread Intervals.” Accordingly, Frame #1 supports a total of 32different request segments.

Given this structure, each request signal is transmitted using a certain16-chip outer code word and a certain 32-chip inner code word.Furthermore, each request signal spans 16 time slots. In each time slot,a particular “chip” of the 16-chip outer code word modulates an entire32-chip inner code word. Just as a simple example, the 32-chip innercode word may be [0, 1, 0, 1, . . . , 0, 1]. Here, modulating this innercode word with an outer code word “chip” having a value of “0” producesa request signal portion [0, 1, 0, 1, . . . , 0, 1]. Modulating the sameinner code word with an outer code word “chip” having a value of “1”produces a request signal portion [1, 0, 1, 0, . . . , 1, 0].

One or more of the access nodes 104, 106, 108, and 110 can send one ormore requests. Each request can be sent in one of the 32 differentrequest segments in a frame. In the example depicted in FIG. 12, arequest in Frame #2 is sent in the request segment shown as inner codechannel 0 and outer code channel 13.

In response, scheduler node 102 broadcasts assignment messages infeedback signal space (not shown). The assignment messages are broadcastto access nodes 104, 106, 108, and 110, to inform the access nodes ofthe assignments made, so that each access node may correctly send datain the assigned scheduled transmission segment(s).

Orthogonal and Non-Orthogonal Request Signal Designs

According to an embodiment of the invention, the request segments may bepositioned such that adjacent request segments may represent orthogonalportions of the request signal space. Adjacent request segments mayrefer to request segments that are located next to one another in time,frequency, and/or code space. Here, orthogonal portions of a signalspace refer to allotments within the signal space that are designed tobe separable from one another, at least under ideal conditions. Forrequest segments defined based on frequency division multiplexing, suchorthogonal requests may be achieved by implementing adjacent requestsegments as non-overlapping frequency channels. For request segmentsdefined based on time division multiplexing, such orthogonal requestsmay be achieved by implementing adjacent request segments asnon-overlapping time slots. For request segments defined based on codedivision multiplexing, such orthogonal requests may be achieved byimplementing adjacent request segments using orthogonal codes, such asWalsh codes as described in previous sections, Walsh codes combined witha common pseudo noise (PN) sequence using modulo 2 addition (Walsh codeswith common PN sequence overlay), Gold codes, and others.

According to an embodiment of the invention, the request segments may bepositioned such that adjacent request segments may representnon-orthogonal portions of the request signal space. Here,non-orthogonal portions of a signal space refer to allotments within thesignal space that are designed to be only partially separable from oneanother. For request segments defined based on frequency divisionmultiplexing, such non-orthogonal requests may be achieved byimplementing adjacent request segments as partially-overlappingfrequency channels. For request segments defined based on time divisionmultiplexing, such non-orthogonal requests may be achieved byimplementing adjacent request segments as partially-overlapping timeslots. For request segments defined based on code division multiplexing,such orthogonal requests may be achieved by implementing adjacentrequest segments using non-orthogonal codes.

A disadvantage associated with use of non-orthogonal request segments isthe reduction in reception performance. When partially-overlappingfrequency channels, partially-overlapping time slots, and/ornon-orthogonal codes are used, the reception of particular requestsegment might capture some energy from adjacent request segment(s), thusreducing reception performance. On the other hand, an advantageassociated with the use of non-orthogonal request segments is that itallows for a more densely packed request signal space. That is, morerequest segments can be packed into a given request signal space.

According to various embodiments of the invention, the extent ofnon-orthogonality is selected for a particular balance of suchadvantages and disadvantages. In one example, non-orthogonal,time-division multiplexed request segments may be implemented as 1 msectime slots, spaced ½ msec apart. That is, each time slot has a length of1 msec. However, the time slots are arranged to overlap one another,such that a new time slot starts every ½ msec. Thus, adjacent time slotshave a ½ msec overlap region. This particular balance is presented asone example and can be adjusted. The amount of overlap can be increased(e.g., to greater than ½ msec), which achieves an even more denselypacked request signal space but further degrades reception performance.Alternatively, the amount of overlap can be decreased (e.g., to lessthan ½ msec), which achieves a less densely packed request signal spacebut improves reception performance. The level of non-orthogonality canbe similarly adjusted for request segments based on frequency divisionmultiplexing (by adjusting overlapping frequency channels) and codedivision multiplexing (by selecting different non-orthogonal codes).

Furthermore, different combinations of multiplexing type andnon-orthogonality technique can be employed. For example, requestsegments may be based on a code division multiplexing employingnon-orthogonal codes and overlapping time slots. This may beimplemented, for instance, using non-orthogonal codes defined over 32msec code intervals (time slots) spaced 16 msec apart. Thus, adjacentcode intervals have a 16 msec overlap region. In such a case,non-orthogonality is attributed to the use of both non-orthogonal codesand overlapping time slots. As another example, request segments may bebased on code division multiplexing employing non-orthogonal codes and acommon code interval. In such a case, non-orthogonality is attributedonly to the use of non-orthogonal codes.

Robust FIFO Scheduling and Efficient FIFO Scheduling

FIG. 5 and other figures depict an assignment message under a “robustFIFO schedule mode,” in accordance with one embodiment of the invention.As mentioned previously, in this mode, each assignment messageexplicitly includes a pair of data: (1) an identifier for the requestand (2) an identifier for the scheduled transmission segment associatedwith the request. In other words, the pairing of a request to anassociated scheduled transmission segment is directly stated in theassignment message. For example, as shown in FIG. 5, the firstassignment message includes the pair of data “REQ 0:7, SCH 13.” Thisindicates that the request sent in the request segment known as “REQ0:7” (the request segment located at Frame 0, Slot 7) has been assignedthe scheduled transmission segment known as “SCH 13” (the scheduledtransmission segment located in Frame 13).

Processing of a message under the “robust FIFO schedule mode” by anaccess node such as 104, 106, 108, and 110 is described below, accordingto one embodiment. Here each access node maintains a local datatransmission queue based on assignment messages received at the firstnode. The local data transmission queue may be a first-in-first-out(FIFO) list of all symbol-level-requests from all access nodes that havebeen assigned to scheduled transmission segments, arranged in the orderof transmission. Each entry in the queue may contain not only anidentifier for a symbol-level request, but also an identifier for thecorresponding scheduled transmission segment. The local datatransmission queue is synchronized to the timing of the rest of thesystem. Thus, as each transmission occurs or is about to occur, theaccess node examines the corresponding entry in the local datatransmission queue. If the access node recognizes the entry as beingassociated with a symbol-level request sent from the access node, theaccess node knows that its turn has come and sends its pending datatransmission. If the access node does not recognize the entry as beingassociated with a symbol-level request set from the access node, theaccess node can assume that it is another access node's turn, and doesnot send a data transmission.

The assignment message may take on a different format under an“efficient FIFO schedule mode,” in accordance with an alternativeembodiment of the invention. In this mode, each assignment message mayonly include one piece of data: an identifier for the request. Thescheduled transmission segment associated with each request is notdirectly stated in the assignment message. Instead, the scheduledtransmission segment associated with each request is inferred.

Processing of a message under the “efficient FIFO schedule mode” by anaccess node such as 104, 106, 108, and 110 is described below, accordingto one embodiment. Again, each access node maintains a local datatransmission queue based on assignment messages received at the firstnode. The local data transmission queue may be a first-in-first-out(FIFO) list of all symbol-level-requests from all access nodes that havebeen assigned to scheduled transmission segments, arranged in the orderof transmission. Each entry in the queue may contain just an identifierfor a symbol-level request. The local data transmission queue issynchronized to the timing of the rest of the system. Thus, as eachtransmission occurs or is about to occur, the access node examines thecorresponding entry in the local data transmission queue. If the accessnode recognizes the entry as being associated with a symbol-levelrequest sent from the access node, the access node knows that its turnhas come and, sends its pending data transmission. If the access nodedoes not recognize the entry as being associated with a symbol-levelrequest set from the access node, the access node can assume that it isanother access node's turn, and does not send a data transmission.

Because a message under the “efficient FIFO schedule mode” does notcontain an identifier for the scheduled transmission segment associatedwith the request, it may be more difficult to keep each access node suchas 104, 106, 108, and 110 in synchronization with the rest of thesystem. For example, if an access node misses an assignment message forsome reason, the local data transmission queue maintained at the accessnode may become out of sync with the actual timing of datatransmissions. To address this potential problem, scheduler node 102 mayperiodically broadcast synchronization messages to the access nodes.This allows each access node to synchronize its local data transmissionqueue with the data transmission queue maintained at scheduler node 102.

Request Detection Error Processing

Undetected requests are handled by request retransmission, according toan embodiment of the invention. Here, an access node such as 104, 106,108, and 110 may transmit a symbol-level request that is not detected bythe intended recipient, such as scheduler node 102. A variety of factorsmay contribute to such a missed detection, such as noise, interference,etc. When the access node senses that a symbol-level request it has senthas not been responded to (e.g., no assignment message is received fromthe scheduler node 102 in response to the symbol-level request), theaccess node may retransmit the symbol-level request. The retransmissiontechnique may also take into account considerations such asretransmission count, latency, and quality of service (QOS). Forexample, the access node may keep track of a retransmission count, suchas the number of times a particular request has been retransmitted. Ifthe retransmission count exceeds a maximum threshold, the request may bediscarded so that no further retransmission is attempted. Also, theaccess node may keep track of a measure of latency, such as the amountof time that has elapsed since the access node began attempting torequest a scheduled transmission segment for a particular message. Ifthe latency exceeds a maximum threshold, the request may be discarded sothat no further retransmission is attempted. Furthermore, the accessnode may take into account a measure of QOS for a request. If therequest is associated with a higher QOS, the request may be given ahigher priority in retransmission.

Request collisions detected by scheduler node 102 may be treated in asimilar manner as undetected requests, according to an embodiment of theinvention. Here, two or more symbol-level requests sent from differentaccess nodes such as 104, 106, 108, and 110 may collide with oneanother. Such collisions may occur if the two or more symbol-levelrequests are sent in the same request segment. If scheduler node 102detects such a collision, it may simply not send an assignment. Thus,the response from scheduler node 102 is the same, regardless of whetherscheduler node 102 misses the symbol-level request or detects that thesymbol-level request was involved in a collision. That is, schedulernode 102 does not send an assignment.

Accordingly, the access node does not need to distinguish between anundetected request versus a detected request collision. As long as theaccess node senses that a symbol-level request it has sent has not beenresponded to (e.g., no assignment message is received from schedulernode 102 in response to the symbol-level request), the access node mayretransmit the symbol-level request. Again, the retransmission techniquemay take into account various considerations such as those listed above.

Request collisions not detected by scheduler node 102 as a collision mayneed to be handled differently. Here, two or more symbol-level requestscollide. However, scheduler node 102 fails to recognize that a collisionhas occurred. Instead, scheduler node 102 treats the collision as avalid symbol-level request. Thus, scheduler node 102 sends an assignmentmessage associating the symbol-level request to a scheduled transmissionsegment. Each access node that sent a symbol-level request involved inthe collision accepts this assignment, and in response sends its owndata transmission. As a result, a collision of multiple datatransmissions can occur in the scheduled transmission segment specifiedin the assignment.

The system may handle such a scenario in a number of ways. In oneembodiment of the invention, receipt of the data transmission is not tobe acknowledged. For example, when an access node such as 104, 106, 108,and 110 sends a data transmission in an assigned scheduled transmissionsegment, the access node does not expect the intended recipient torespond by sending an acknowledgement (ACK) message to confirmsuccessful receipt of the data transmission. Here, the access node doesnot inquire into whether the data transmission is successfully received.If it is not successfully received, the access node does not take anyaction to re-send the data transmission.

In an alternative embodiment of the invention, receipt of the datatransmission is to be acknowledged. For example, when an access nodesuch as 104, 106, 108, and 110 sends a data transmission in an assignedscheduled transmission segment, the access node expects the intendedrecipient to respond by sending an acknowledgement (ACK) message toconfirm successful receipt of the data transmission. Here, if the datatransmission is not successfully received, as indicated by the lack ofan ACK message from the intended recipient, the access node re-sends thedata transmission. The process of re-sending the data transmission maybe similar to that of the original transmission. That is, the accessnode may first send a symbol-level request, then receive an assignmentassociating the symbol-level request to a particular scheduledtransmission segment, and finally send the data transmission in theassigned scheduled transmission segment.

Contention Requests

According to an embodiment of the present invention, when an access nodesuch as 104, 106, 108, and 110 sends a symbol-level request, the accessnode randomly selects a request segment from the plurality of availablerequest segments. As mentioned previously, the request segments may beorganized in the request signal space based on time-divisionmultiplexing, frequency-division multiplexing, code-divisionmultiplexing, and/or other multiplexing techniques.

For example, the request segments can be organized in the request signalspace based on code-division multiple access (CDM). A code such as aWalsh code may be used. Each request segment corresponds to a codechannel that can be extracted from the request signal space by applyinga particular code word. By utilizing CDM, the request segment isextended in time and spread in frequency. That is, without applyingcode-division multiple access, the request segment may be confined to ashorter time slot and narrower frequency bandwidth. Thus, the energy ofa request can be spread out over a longer time slot and over a widerfrequency bandwidth. This allows the transmitter to use less power.Specifically, the arrangement captures the otherwise unused power (powerin unoccupied time slots and unoccupied frequency bandwidths) in a lowload factor request channel. Use of CDM in the request signal space alsoprovides a signal-to-noise ratio (SNR) gain over transmissions in thescheduled transmission signal space. In certain implementations, thisgain can be in the range of 9 to 30 dB, or even more. The use of CDM asapplied to request segments can significantly improve request detectionperformance.

FIG. 13 presents a more detailed example of a system with a contentioncode request channel, according to an embodiment of the invention. Here,the system utilizes a scheduled transmission data length of 800 channelsymbols, which is equivalent to 100 bytes in this case. The averageinterval of scheduled transmissions from each node is 8 seconds. Theaverage bandwidth of scheduled transmissions from each node is 100symbols/sec. The ratio of request symbols to scheduled transmissionsymbols is 8/100. In other words, every 64 request symbols correspondsto 800 scheduled transmission symbols. A 64 chip spread code is used isthat provides a 18 dB SNR spread gain. The request channel load factoris 1/64. The resulting collision probability of requests is 1.55%.

Polled Requests

According to another embodiment of the present invention, when an accessnode such as 104, 106, 108, and 110 sends a symbol-level request, theaccess node utilizes a defined schedule to select a request segment fromthe plurality of available request segments. The defined schedulespecifies the request segments available to each access node. In thissense, the defined schedule “polls” each access node to send its requestin the appropriate request segment. Again, the request segments may beorganized in the request signal space based on time-divisionmultiplexing, frequency-division multiplexing, code-divisionmultiplexing, and/or other multiplexing techniques. In particular, theuse of code-division multiplexing in the request signal space isassociated with significant benefits, as discussed previously.

Generally speaking, a polled request channel has both advantages anddisadvantages when compared to a contention request channel. Oneadvantage is that there is no possibility of collision. This is becausethe defined schedule does not allot the same request segment to twodifferent access nodes. A disadvantage of the polled request channel isadded latency. When an access node adopting a polled request channel isready to send a request, it might not be able to do so right away.Instead, the access node may have to wait for the next opportunity whenone of its request segments becomes available according to the definedschedule. Such delays contribute to the added latency of a polledrequest channel.

According to one embodiment of the invention, the defined schedule isfixed. For example, the nodes in the system may be preprogrammed tofollow a particular defined schedule of request segments. According toanother embodiment of the invention, the defined schedule is dynamicallyaltered. For example, the scheduler node 102 may change the definedschedule of requests segments during operation, in response to changingconditions. The scheduler node 102 may then broadcast the newly definedschedule to access nodes 104, 106, 108, and 110.

FIG. 14 presents a more detailed example of a system with a polled coderequest channel, according to an embodiment of the invention. Here, thesystem utilizes a scheduled transmission data length of 800 channelsymbols, which is equivalent to 100 bytes in this case. The averageinterval of scheduled transmissions from each node is 8 seconds. Theaverage bandwidth of scheduled transmissions from each node is 100symbols/sec. The request poll interval for each node is set at 125 msec,which corresponds to 8 request opportunities per second per node. Theratio of request symbols to scheduled transmission symbols is 8/100. Inother words, every 64 request symbols corresponds to 800 scheduledtransmission symbols. A 64 chip spread code is used is that provides a18 dB SNR spread gain. The collision probability of requests is 0% ifthe system operates as intended.

Request Signal Designs for Multiple Service Types

According to an embodiment of the present invention, multiple servicesare offered. Accordingly, multiple categories of scheduled transmissionsegments are defined in the scheduled transmission signal space. Eachcategory of scheduled transmission segments may correspond to adifferent service type. For example, as described previously, FIG. 6presents a two-service TDM request and scheduled transmission scheme. Inthis case, there are two categories of scheduled transmission segments:long request segments for data transmissions of a first length (long)and short request segments for data transmissions of a second length(short). FIG. 6 merely illustrates a simple example. The service typesmay be based on factors other than data transmission length. Also, theremay be more than two types of service.

To request a scheduled transmission of a particular service type, anaccess node such as 104, 106, 108, and 110 sends a request that somehowindicates the service type desired. According to various embodiments ofthe invention, such a request may be implemented as a one-symbol (singlesymbol) request or an N-symbol (multiple symbols) request.

One-symbol requests are described in more detail below. Here, the accessnode conveys the service type desired using only a one-symbol request.According to an embodiment of the invention, this is accomplished bydividing the request signal space into multiple categories of requestsegments. An access node can indicate the type of service requested bysending the request in a request segment belonging to the appropriatecategory. For example, in the case of two-service system shown in FIG.6, some request segments are designated as “long” request segments,while other request segments are designated as “short” request segments.To request a “long” scheduled transmission, an access node such as 104,106, 108, and 110 simply sends a request in one of the “long” requestsegments. Such a technique has numerous advantages. One is that multipleservice types (e.g., greater than two) can be supported. Another is thata flexible allocation of the request signal space to different servicetypes can be achieved. For example, in the two-service system shown inFIG. 6, the make-up of the request signal space, in terms of allocationfor “long” request segments versus allocation for “short” requestsegments, can be flexibly changed. Furthermore, this make-up of therequest signal space can be dynamically adjusted while the system is inoperation.

N-symbol requests are described in more detail below. Two separateembodiments are presented. According to a first embodiment, a requestcomprising N symbols is non-coherently detected. That is, the recipientof the request (e.g., scheduler node 102, other access nodes such as104, 106, 108, and 110) receives the request as a non-coherent signal.For example, the recipient may utilize an envelope detector that onlydetects each symbol of the request as either an “on” or “off.” Generallyspeaking, such an N-symbol request can specify one out of 2^(N)−1service types. In one implementation where each request comprises N=2symbols, the request may support 3 different types of services asfollows. If symbol 1 is “on” and symbol 2 is “off,” then service type 1is indicated. If symbol 1 is “off” and symbol 2 is “on,” then servicetype 2 is indicated. If symbol 1 is “on” and symbol 2 is “on,” thenservice type 3 is indicated. Of course, if symbol 1 is “off” and symbol2 is “off,” then no request is sent.

FIG. 15 is a plot of expected envelop detector performance underdifferent noise levels, according to an embodiment of the invention.According to various embodiments of the invention, an envelop detectormay be implemented to detect requests sent in the request signal space.This includes detection of one-symbol requests as well as N-symbolrequests, as discussed above. Designs of such envelop detectors arewell-know in the art. FIG. 15 shows a number of performance curves,known in the art as receiver operating characteristic (ROC) curves. Eachcurve represents the expected performance of the envelop detector for aparticular noise level, which is expressed in terms of a ratio of energyper symbol over noise (Es/No). Each curve demonstrates the trade offthat can be expected at a particular noise level between (1) probabilityof missed detection and (2) probability of false alarm for the detector.Adjusting the detection threshold of the detector moves the performanceof the detector along the curve. Any point on the curve represents aparticular tradeoff between probability of missed detection andprobability of false alarm. As can be expected, the overall performanceof the envelop detector degrades as noise level increases.

According to a second embodiment, a request comprising N symbols iscoherently detected. That is, the recipient of the request receives therequest as a coherent signal. Designs of such coherent detectors arewell-know in the art. The recipient may demodulate the request accordingto a particular modulation and/or coding scheme. One example is a systembased on a differentially coherent detection scheme. Here, M-arydifferential phase shift keying (M-DPSK) modulation is used. An N-symbolrequest can specify one of M^(N-1) different service types, with onesymbol used to provide a phase reference. Furthermore, the M-DPSK signalmay also be block-coded to provide error correction. Of course, this isjust one example. In other implementations, different modulation andcoding schemes may be used. The particular choice of modulation andcoding scheme may be depend on channel conditions, such as the signal tonoise ratio (SNR) encountered.

Default Assignment of Scheduled Transmission Segments

According to an embodiment of the invention, the scheduled transmissionsignal space comprises a plurality of scheduled transmission segmentsthat includes at least one default use segment available for use by adefault node or a plurality of default nodes if not assigned to anysymbol-level request. Such a technique allows for a flexible use oftransmission segments that do not become assigned as a result of aspecific symbol-level request.

In one implementation, a two-tier assignment technique is employed. Thefirst tier is a “priority tier” of assignments. This may comprise anassignment associating a symbol-level request with a scheduledtransmission segment, as discussed in other sections of the presentdisclosure. A second tier is a “space available” tier of assignments. Inthis tier, one or more scheduled transmission segments are assigned to adefault entity. The default entity is given conditional use of thesescheduled transmission segments. The condition is that, if the scheduledtransmission segments are not assigned in the priority tier (thus theyare available), the default entity may go ahead and use the scheduledtransmission segments. In other words, the second tier allows forsecondary assignments of scheduled transmission segments.

Each of these second-tier assignments may be specified for a particulartime duration. Typically, the time duration is relatively long withrespect to the first-tier assignment activities of the system. Just asone example, in a particular system, numerous first-tier assignments maytake place in a matter of seconds. By contrast, a second-tier assignmentmay assign one or more scheduled transmission segments to a defaultentity for a time duration of minutes, hours, or days. Also, thesecond-tier assignments may partition the scheduled transmission signalspace in a different way than the first tier of assignments. Forexample, a second-tier assignment may assign a large block of scheduledtransmission segments for default use by a default entity.

In one embodiment, the default entity comprises a default node. Here,the default node enjoys use of the one or more scheduled transmissionsegments if they are not assigned in the first tier. The default nodemay use the scheduled transmission segments to perform file transfers,certain low priority data flows, etc.

In another embodiment, the default entity comprises a group of defaultnodes. Here, the group of default nodes shares the default use of theone or more scheduled transmission segments. The scheduled transmissionsegments may be allocated as a block of the signal space. The group ofdefault nodes may share the default use of the block on a contentionbasis. For example, each member of the group of default nodes maycompete with the other members for the right to use the entire block ofsignal space. Various contention access protocols may be implemented,such as Slotted Aloha and others.

In yet another embodiment, the default entity is selected from aplurality of default entities including at least one default node and atleast one group of default nodes. Thus, in one system, the second tiermay include a mix of default nodes and groups of default nodes. Aparticular second-tier assignment may assign the default use of a blockof signal space to a default node or to a group of default nodes.

Successive Scheduled Requests

In response to an initial symbol-level request sent in an initialrequest segment from access nodes 105, 106, 108, and 110, scheduler node102 may assign one or more additional request segments as follow-uprequest segments to the initial symbol-level request, according to anembodiment of the invention. For example, scheduler node 102 maybroadcast an assignment message to associate the initial request segmentwith the one or more follow-up request segments. This may be performedin an anonymous manner with respect to the identity of the access nodethat sent the initial request, as described previously. The one or morescheduled follow-up request segments may be used in different ways. Twosuch uses are described below.

In one embodiment, the follow-up request segments are used to resolvecollisions. When scheduler node 102 receives the initial symbol-levelrequest, it may determine that a collision may have occurred. Just as anexample, assume two access nodes, 104 and 106, each sends an initialrequest in the same initial request segment. This causes a collision.Even if scheduler node 102 can properly decode the initial request, itremains ambiguous as to which access node should be the deemed theoriginator of the request. If scheduler node 102 does not resolve thecollision and instead simply sends an assignment message associating theinitial request with a scheduled transmission segment, both access nodes104 and 106 may accept the assignment and attempt to send a datatransmission in the scheduled transmission segment. The likely result isthat the signals will be jumbled, and neither data transmission can beproperly received.

Thus, instead of just sending an assignment associating the initialrequest segment with a scheduled transmission segment, scheduler node102 may send an assignment associating the initial request segment withone or more follow-up request segments. These follow-up request segmentsprovide an opportunity for access nodes involved in a collision to sendsecondary requests that may avoid collision with one another.

Continuing with the above example of a collision involving access nodes104 and 106, scheduler node 102 may detect the possible collision andsend an assignment that associates the initial request with 32 follow-uprequest segments. Access node 104 responds by randomly selecting 1 outof the 32 follow-up request segments and sending a secondary request inits randomly selected request segment (e.g., request segment 8). Accessnode 106 also responds by randomly selecting 1 out of the 32 follow-uprequest segments and sending a secondary request in its randomlyselected request segment (e.g., request segment 13).

There is a high probability (31/32) that access nodes 104 and 106 wouldrandomly select different ones of the 32 possible follow-up requestsegments to send their respective secondary requests. If this scenariooccurs (more likely), the collision has been resolved. That is, nowaccess nodes 104 and 106 have sent secondary requests in distinctrequest segments. Scheduler node 102 can simply process the secondaryrequests separately—by assigning a scheduled transmission segment to thesecondary request sent from access node 104, and separately assigninganother scheduled transmission segment to the secondary request sentfrom access node 106.

Of course, there is a low probability (1/32) that access nodes 104 and106 would happen to select the same one out of the 32 possible follow-upsegments to send their respective secondary requests. If this scenariooccurs (less likely), the collision remains unresolved. Here, schedulernode 102 may repeat the procedure over again, in order to resolve thecollision. That is, scheduler node 102 may send yet another assignmentto assign another set of 32 follow-up request segments. Access nodes 104and 106 would again separately make random selections out of the 32follow-up request channels, and so on. This process may repeat until thecollision is resolved or until a termination condition is reached.

While the example discussed above involves two access nodes 104 and 106,the approach is applicable in collision situations involving more thantwo nodes. Also, the probability of collision resolution in eachiteration can be modified by changing the number of follow-up requestsegments assigned (e.g., to a number greater than or less than 32).

In another embodiment, the follow-up request segments are used to sendadditional information relating to a request. An access node such as104, 106, 108, and 110 may send an initial symbol-level request. Uponreceiving the initial symbol-level request, scheduler node 102 may sendan assignment associating the initial request segment with one or morefollow-up request segments. Here, the follow-up request segments providean opportunity for the access node to provide additional informationthat supplements the initial symbol-level request. For example, if theinitial symbol-level request does not indicate a particular type ofservice requested, scheduler node 102 may assign one or more follow-uprequest segments so that the access node may send additional signals inthe follow-up request segments to specify the type of service requested.

Piggyback Requests

According to an embodiment of the invention, a request may be sent froman access node such as 104, 106, 108, and 110 as a “piggyback request.”Here, a piggyback request refers to a request that is included in aportion of a data transmission sent from a node in a scheduledtransmission segment. Thus, unlike a request sent in a request segmentin the request signal space, a piggyback request is sent as part of adata transmission, such as in a header of the data transmission, sent ina scheduled transmission segment in the scheduled transmission signalspace. This technique may improve efficiency by allowing a request to“hitch a ride” on a scheduled transmission, thus obviating the use aseparate transmission in the request signal space.

Operation of piggyback requests can be viewed in the context of asequence of data transmissions sent, in respective scheduledtransmission segments, from an access node. The sequence of datatransmissions may include a current data transmission that the accessnode is prepared to send. The sequence of data transmissions may alsoinclude a subsequent data transmission that has yet to be assigned ascheduled transmission segment. To request a scheduled transmissionsegment for the subsequent data transmission, the access node may send apiggyback request in a portion of the current data transmission.

The piggyback request may be included in the current data transmissionin various ways. For example, the piggyback request may be part of aheader in the current data transmission. In one implementation, thepiggyback request comprises a request count in the header of the currentdata transmission.

In one embodiment of the invention, a piggyback request may serve as aninitial request for the subsequent data transmission. For example, thepiggyback request may be the only request sent from the access node forrequesting assignment of a scheduled transmission segment for thesubsequent data transmission. In this case, there is no separate requestsignal sent in the request signal space for the subsequent datatransmission. That is, the subsequent data transmission relies on thepiggyback request to obtain an assignment of an appropriate scheduledtransmission segment.

The access node may make a burst time decision to select between using apiggyback request versus using a symbol-level request in the requestsignal space, according to an embodiment of the invention. The bursttime decision occurs around the time when the current data transmissionis about to be sent. The selection may be based on the expected latencyof the current data transmission. The selection may also be based oncertain quality-of-service (QOS) considerations associated with the datatransmission to be scheduled, i.e., the subsequent data transmission.Just as a simple example, a particular QOS requirement for thesubsequent data transmission may be that requests are to be sent in lessthan 100 msec. If the expected latency of the current data transmissionexceeds 100 msec, the access node may choose to use asymbol-level-request in the request signal space, instead of thepiggyback request.

In one embodiment of the invention, a piggyback request may serve as aredundant request corresponding to a previously sent request. Here, eachaccess node sends not only (1) a request in the request signal space,but also (2) a redundant piggyback request in the scheduled transmissionsignal space. The piggyback request may include a location identifierfor the associated request in the request signal space. Thus, eachrequest sent by an access node in the request signal space has a back upin the form of an associated redundant piggyback request.

One use of such redundant piggyback requests is detection of collisionsbetween multiple requests in a request segment. Different nodes such asaccess nodes 104, 106, 108, and 110, may send requests in the samerequest segment in the request signal space, causing a collision. Ifeach access node also sends an associated redundant piggyback request asbackup, the redundant requests may be processed to detect the collision.Another use of such redundant piggyback request is for detection ofmissed requests.

FIG. 16 depicts use of piggyback requests for request collisiondetection and missed request processing, according to an embodiment ofthe invention. The figure shows possible events at a node such asscheduler node 102, and corresponding action taken.

First, upon receiving a request in the request signal space, scheduler102 assigns a scheduled transmission segment to the request. Second,upon receiving a single (sole) piggyback request associated with aparticular request detected in the request signal space, scheduler 102ignores the piggy back request. Here, receipt of a single piggybackrequest indicates that only one access node sent the request in therequest signal space, and thus no collision occurred. Third, uponreceiving multiple piggyback requests associated with a particularrequest detected in the request signal space, scheduler 102 assigns aseparate scheduled transmission segment to each piggyback request. Here,receipt of multiple piggyback requests that are all associated with thesame request detected in the request signal space indicates that morethan one of the access nodes sent the request, and thus a collisionoccurred. Thus, scheduler node 102 resolves the conflict by assigningeach piggyback request a separate scheduled transmission segment.Fourth, upon receiving a piggyback request for an undetected request inthe request signal space, scheduler 102 assigns a scheduled transmissionsegment to the piggyback request. Here, the piggyback request isassociated with a request that was sent in the request signal space, butsomehow was not detected. Here, the piggyback request serves its purposeas the backup request. Accordingly scheduler 102 assigns a scheduledtransmission segment to the piggyback request.

Dynamic Partitioning of Request Signal Space and Scheduled TransmissionSignal Space

According to an embodiment of the present invention, the allocation ofthe shared communication medium between the request signal space and thescheduled transmission signal space can be dynamically altered. Just asa simple example, a system may dynamically alter the allocation of theshared communication medium, from an allocation as illustrated in FIG.5, to an allocation as illustrated in FIG. 6. As discussed previously,FIG. 5 depicts a particular allocation of the shared communicationmedium based on frames of TDM time slots. FIG. 6 depicts a differentallocation of the shared communication medium, also based on frames ofTDM time slots. During operation, a system may dynamically switch fromthe allocation shown FIG. 5 to the allocation shown in FIG. 6. A nodesuch as schedule node 102 may broadcast one or more control messages toother nodes such as access nodes 104, 106, 108, and 110 in order tonotify them of the change in the allocation. Once the other nodes havebeen notified of the change, the system can switch over to operationaccording to the new allocation of the shared communication medium.

Two-Step Request Occupying Request Segment and Scheduled TransmissionSegment

According to an embodiment of the invention, access nodes 104, 106, 108,and 110 may send a two-step request for transmission. Such a two-steprequest is made up of a first step and a second step. The first step maysimply be a symbol-level request sent from the access node, as describedpreviously. When an assignment is obtained associating the symbol-levelrequest to a scheduled transmission segment, the access node can send adata transmission in the scheduled transmission segment. Here, thesecond step may be the data transmission sent in the scheduledtransmission segment. In other words, instead of using the scheduledtransmission segment assigned to a request for sending regular data, theaccess node uses the scheduled transmission segment to send the secondstep of the request. The second step may further define the request. Therequest may be a more elaborate request for service. For example, thesecond step may specify a particular service type or other informationthat provides additional details on the request being made.

Multiple Requests for Priority Access

According to an embodiment of the invention, an access node such as 104,106, 108, and 110 may send more than one request when attempting toobtain an assignment to schedule only one data transmission. Ordinarily,when the access node has a data transmission to send, the access nodesends one request in order to obtain an assignment for a scheduledtransmission segment. Here, however, the access node may send out two ormore requests. In other words, the access node may send an extrarequest(s) as backup. This may be done for the purpose of increasing thelikelihood of an assignment. For example, if scheduler 102 somehow failsto detect one of the requests, the other request(s) may still bedetected. For each detected request, scheduler 102 may send anassignment that assigns the detected request to a scheduled transmissionsegment.

Of course, this can lead to multiple assignments of separate scheduledtransmission segments, reserved for just one data transmission.According to one embodiment of the invention, the access node does notwaste such extra scheduled transmission segments. Instead, the accessnode utilizes such extra scheduled transmission segment for sendingother data transmissions.

No Scheduler Mode and Other Assignment Modes

Different assignment modes may be adopted according to variousembodiments of the invention. These may include a “scheduler mode,” a“no scheduler mode,” and a “hybrid mode.” Here, a “scheduler mode”refers to a mode of operation in which access nodes rely on a schedulernode to determine the assignment of scheduled transmission segments.Previous sections have described examples under the “scheduler mode.”

FIG. 17 presents a simplified network operating under a “no schedulermode,” according to one embodiment of the invention. A plurality ofaccess nodes 1404, 1406, 1408, and 1410 are shown that utilize a sharedcommunication medium. Instead of depending on a scheduler node toreceive requests and determine the proper assignment of scheduledtransmission segments, each access node 1404, 1406, 1408, and 1410independently determines the proper assignment of scheduled transmissionsegments. Here, it is assumed that all access nodes follow the samerules for determining assignments, and all access nodes can detect allrequests. If this is the case, then the same assignment of scheduledtransmission segments would be generated at each access node. That is,each access node would independently generate the same assignment. Assuch, there would be no need for a dedicated scheduler node. Also, therewould be no need for assignment messages to be sent. Each access nodewould be able to locally determine the proper assignment on its own.Consequently, a feedback signal space may not need to be provided forsending any assignment messages.

A “hybrid mode” is described below in accordance with anotherembodiment. Referring back to FIG. 1, under this mode, each access nodesuch as 104, 106, 108, and 110 receives both (1) assignment messagesfrom a scheduler node such as 102 and (2) requests from the other accessnodes. Here, each access node does independently determine the properassignment of scheduled transmission segments based on requests receivedfrom other nodes. However, in making the determination, the access nodealso takes into account the assignment messages received from thescheduler node. By utilizing both sources of information, each accessnode can make a more robust determination regarding what is the properassignment of scheduled transmission segments.

According to yet another embodiment of the invention, a system maycontain a mixture of access nodes operating under different assignmentmodes. Some of the access nodes in the system may operate under a“scheduler mode.” Some of the access nodes in the system may operateunder a “no scheduler mode,” as discussed above. Finally, some of theaccess nodes in the system may operate under a “hybrid mode, asdiscussed above.

While the present invention has been described in terms of specificembodiments, it should be apparent to those skilled in the art that thescope of the present invention is not limited to the described specificembodiments. The specification and drawings are, accordingly, to beregarded in an illustrative rather than a restrictive sense. It will,however, be evident that additions, subtractions, substitutions, andother modifications may be made without departing from the broaderspirit and scope of the invention as set forth in the claims.

1. A method for conducting communications over a shared communication medium involving a plurality of nodes, the method comprising: (a) transmitting a request from a first node in the plurality of nodes, the shared communication medium organized to include a request signal space and a scheduled transmission signal space, the request signal space including a plurality of request segments each having a different location within the request signal space, the scheduled transmission signal space including a plurality of scheduled transmission segments each having a different location within the scheduled transmission signal space, the request sent in a request segment that is one of the plurality of request segments; (b) obtaining an assignment, in a first tier of assignments, associating the request with a scheduled transmission segment selected from the plurality of scheduled transmission segments, the assignment taking into account location of the request within the request signal space; (c) from the first node, transmitting a data transmission in the scheduled transmission segment associated with the request in accordance with the assignment; and (d) wherein the plurality of scheduled transmission segments includes at least one default use segment, the at least one default use segment being available for use by a default entity if the at least one default use segment is not assigned in the first tier of assignments.
 2. The method of claim 1 wherein an assignment, in a second tier of assignments, associates the at least one default use segment with the default entity.
 3. The method of claim 2 wherein the assignment in the second tier of assignments remains in effect for a specified time duration.
 4. The method of claim 2 wherein the assignment in the second tier of assignments defines a block of scheduled transmission segments as the at least one default use segment.
 5. The method of claim 1 wherein the default entity is a default node selected from the plurality of nodes.
 6. The method of claim 5 wherein the default node utilizes the at least one default use segment for file transfers.
 7. The method of claim 5 wherein the default node utilizes the at least one default use segment for low priority data flows.
 8. The method of claim 1 wherein the default entity is a group of default nodes selected from the plurality of nodes.
 9. The method of claim 8 wherein the group of default nodes share the at least one default use segment on a contention access basis.
 10. The method of claim 1 wherein the default entity is selected from a plurality of default entities including at least one default node and at least one group of default nodes.
 11. A method for conducting communications over a shared communication medium involving a plurality of nodes including a first node and a second node, the method comprising: (a) at the second node, receiving a request from the first node, the shared communication medium organized to include a request signal space and a scheduled transmission signal space, the request signal space including a plurality of request segments each having a different location within the request signal space, the scheduled transmission signal space including a plurality of scheduled transmission segments each having a different location within the scheduled transmission signal space, the request sent in a request segment that is one of the plurality of request segments; (b) at the second node, making an assignment, in a first tier of assignments, associating the request with a scheduled transmission segment selected from the plurality of scheduled transmission segments, the assignment taking into account location of the request within the request signal space, and transmitting a corresponding assignment message; (c) at the second node, receiving a data transmission from the first node in the scheduled transmission segment associated with the request; and (d) wherein the plurality of scheduled transmission segments includes at least one default use segment, the at least one default use segment being available for use by a default entity if the at least one default use segment is not assigned in the first tier of assignments.
 12. An apparatus for conducting communications over a shared communication medium involving a plurality of nodes, comprising: (a) a first node capable of sending a request over the shared communication medium, the shared communication medium organized to include a request signal space and a scheduled transmission signal space, the request signal space including a plurality of request segments each having a different location within the request signal space, the scheduled transmission signal space including a plurality of scheduled transmission segments each having a different location within the scheduled transmission signal space, the first node capable of sending the request in a request segment that is one of the plurality of request segments; (b) wherein the first node is capable of obtaining an assignment, in a first tier of assignments, associating the request with a scheduled transmission segment selected from the plurality of scheduled transmission segments, the assignment taking into account location of the request within the request signal space; (c) wherein the first node is capable of sending a data transmission in the scheduled transmission segment associated with the request in accordance with the assignment; and (d) wherein the plurality of scheduled transmission segments includes at least one default use segment, the at least one default use segment being available for use by a default entity if the at least one default use segment is not assigned in the first tier of assignments.
 13. The apparatus of claim 12 wherein an assignment, in a second tier of assignments, associates the at least one default use segment with the default entity.
 14. The apparatus of claim 13 wherein the assignment in the second tier of assignments remains in effect for a specified time duration.
 15. The apparatus of claim 13 wherein the assignment in the second tier of assignments defines a block of scheduled transmission segments as the at least one default use segment.
 16. The apparatus of claim 12 wherein the default entity is a default node selected from the plurality of nodes.
 17. The apparatus of claim 16 wherein the default node utilizes the at least one default use segment for file transfers.
 18. The apparatus of claim 16 wherein the default node utilizes the at least one default use segment for low priority data flows.
 19. The apparatus of claim 13 wherein the default entity is a group of default nodes selected from the plurality of nodes.
 20. The apparatus of claim 19 wherein the group of default nodes share the at least one default use segment on a contention access basis.
 21. The apparatus of claim 12 wherein the default entity is selected from a plurality of default entities including at least one default node and at least one group of default nodes.
 22. An apparatus for conducting communications over a shared communication medium involving a plurality of nodes, comprising: (a) a second node capable of receiving a request from a first node, the shared communication medium organized to include a request signal space and a scheduled transmission signal space, the request signal space including a plurality of request segments each having a different location within the request signal space, the scheduled transmission signal space including a plurality of scheduled transmission segments each having a different location within the scheduled transmission signal space, the second node capable of receiving the request in a request segment that is one of the plurality of request segments; (b) wherein the second node is capable of making an assignment, in a first tier of assignments, associating the request with a scheduled transmission segment selected from the plurality of scheduled transmission segments, the assignment taking into account location of the request within the request signal space, and sending a corresponding assignment message; (c) wherein the second node is capable of receiving a data transmission from the first node in the scheduled transmission segment associated with the request; and (d) wherein the plurality of scheduled transmission segments includes at least one default use segment, the at least one default use segment being available for use by a default entity if the at least one default use segment is not assigned in the first tier of assignments. 